Our main goal was to understand how myelin forms. To realize this goal we studied the mechanisms of myelin formation during the development of mice and zebrafish. We analyze the factors that determine whether and to what extent an axon will be myelinated or not, the forces that drive myelin around the axon, the structural basis of myelin plasticity and the mechanisms of myelin turnover in the adult. To study the mechanism of myelin biogenesis, we combined morphological and molecular analyses in mice and zebrafish, we found that adhesion molecules of the paranodal axo-glial junction and the internodal segment work synergistically using overlapping functions to regulate axonal targeting and myelin wrapping. In the absence of these adhesive systems, axonal recognition by myelin is impaired with myelin being mistargeted on top of previously myelinated fibers, around neuronal cell bodies and above nodes of Ranvier. In addition, myelin wrapping is disturbed with the leading edge moving away from the axon and in between previously formed layers. These data show how two adhesive systems function together to guide axonal ensheathment and myelin wrapping, and provide a mechanistic understanding of how the spatial organization of myelin is achieved. In addition, we identified BCAS1+ oligodendrocytes that represent a oligodendroglial subtype that segregates from OPCs and mature oligodendrocytes in mice and humans. These cells represent a newly generated and early oligodendroglial subtype that is transiently present during the active phase of myelination and can, thus, be used to report areas of active myelination and remyelination. By quantifying the density of BCAS1+ cells, we found that active myelin formation peaks in the first year of postnatal corpus callosum development, whereas lifelong myelination was observed in the frontal human cortex.
The aim was to study the turnover of myelin. Myelin is formed by oligodendrocytes as a multilamellar structure that encloses segments of axons in the CNS. Once myelin is laid down, it is unknown to what extent the sheaths require maintenance and remodeling. Myelin membrane components are metabolically relatively stable with half-lives on the order of several weeks to months. Nevertheless, protein/lipid turnover is, in general, necessary to replace potentially impaired molecules with new functional copies in order to combat functional decline. We find that myelin gradually breaks down and degraded fragments are subsequently cleared by microglia. Myelin fragmentation increases with age and leads to the formation of insoluble, lipofuscin-like lysosomal inclusions in microglia. Our study proposes that age-related myelin fragmentation is substantial leading to lysosomal storage and contributing to microglia senescence and immune dysfunction in aging.
We also studied myelin clearance during following demyelinating injury. Once myelin is damaged disrupted myelin is phagocytosed by resident microglia or infiltrating macrophages. Several studies have shown that such initial immune responses are actually necessary to allow for repair. Once inflammation begins to subside, cells switch to regenerative phenotypes and/or start to resolve from the lesion. Currently, we do not understand which factors determine the fate of MS lesions. To begin to resolve this question we studied the biology of phagocytes during de- and remyelination in mice. We made the surprising finding that the self-limiting inflammatory response, which is necessary for remyelination to occur, is maladaptive in the CNS of aged mice. We found that cholesterol-rich myelin debris overwhelms the efflux capacity of phagocytes resulting in a phase transition of free cholesterol into crystals, which induces lysosomal rupture and inflammasome stimulation. These studies identified the cellular cholesterol efflux pathway as the bottleneck for myelin regeneration in the CNS.
Myelin is metabolically active and capable of communicating with the underlying axon. To be functionally connected to the neuron, oligodendrocytes maintain non-compacted myelin as cytoplasmic nanochannels. We used high-pressure freezing for electron microscopy to study these cytoplasmic regions within myelin close to their native state. We identified 2,′3′-cyclic nucleotide 3′-phosphodiesterase (CNP), an oligodendrocyte-specific protein previously implicated in the maintenance of axonal integrity, as an essential factor in generating and maintaining cytoplasm within the myelin compartment. We provide evidence that CNP directly associates with and organizes the actin cytoskeleton, thereby providing an intracellular strut that counteracts membrane compaction by myelin basic protein. Our study provides a molecular and structural framework for understanding how myelin maintains its cytoplasm to function as an active axon-glial unit. We hypothesize that a system of cytoplasm-rich channels, bidirectionally connecting the oligodendroglial cell body with the inner adaxonal tongue of myelin, are necessary to provide metabolic support, to maintain functional axon-glial units over long time, and to regulate myelin thickness within active neuronal circuits.
